It is well known in the plastic molding art to use pressurized fluid in conjunction with the injection molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the molten resin.
Pressurized fluid serves many purposes. First, it allows the article so formed to have hollow interior portions which result in weight and material savings. Secondly, it minimizes the molded in stresses by eliminating high second stage injection pressure. This also reduces part weight in that the gas is used to fill out the part. Thirdly, the pressurized fluid applies outward pressure to force the plastic against the surfaces of the mold cavity while the article is setting up. This enhances surface quality by eliminating sink marks, especially in areas of the article having thicker plastic sections, such as structural ribs or bosses.
Presently, there are two primary methods employed to relieve the internal pressure in a hollow-shaped body while still located in its respective mold. One method is to relieve the gas through the gas-injection supply through the use of valving. One problem with this approach is that the orifice becomes clogged with resin or other contaminants rendering the orifice non-functional and thereby disrupting normal production.
The second method is commonly called sprue break. This method is performed by disengaging the injection nozzle from the sprue bushing of the mold, thereby exposing the gas channel in the sprue to the atmosphere.
The Friederich U.S. Pat. No. 4,101,617 discloses a method for injection molding of hollow-shaped bodies wherein the pressure is relieved directly into the part. The Friederich patent discloses a sprue break as well as opening the interior of the hollow body to relieve the internal pressure.
The Hendry U.S. Pat. No. 4,474,717 discloses a fluid-assisted injection molding method and system including a probe having a head with one or more fluid orifices. In one embodiment the probe is retractable. Pressure within the hollow-shaped body is relieved through the probe.
The U.K. Patent Application No. 2,139,548 discloses an injection molding process wherein pressurized fluid is injected into a stream of plastic material at one or more selected positions in the mold space through a passageway which projects into the path of the molten resin. Pressure within the resulting hollow-shaped body is relieved through the passageway.